Stressed, secret half sister!

Give you a bit of background, my dad has always worked away.. Army and then security work, a job in Serbia amongst other places, my mum at home with me and my brother, always had a happy life and dad's work circumstances were never an issue.

Today I read a post on Facebook saying how fb filters through messages and puts the unwanted ones in a secret folder without telling us.. Knowing this info I found the folder.. Lots of pervy people messaging me, fake accounts wanting to make friends etc, but then I click on this one from 2014, a 17yo(at the time) girl who lives in Sweden saying that she's my half sister, her mum met my dad in 1996, (5 years after I was born, 2 years after my brother), she and dad then have met once when he was in Serbia and that she would like to meet me and that she thought I should know. To top it off she added a photo of herself and my dad to the email, she very much looks like me, I'm so angry, shocked about all this!

One, why in gods name did I open the bloody email and not just delete it like I had the others...? Angry with myself for that!Two, what the bloody hell do I do now? I think my best possible option is to not say a thing, be quietly angry with my dad, move on and slowly forget about it.I don't know if my mum knows, it will completely break her if she doesn't, I don't know if I owe it to her to tell her, I love her so much, her and my dad have one of those relationships that I look up to (not sure about that anymore) however she does rely on my dad, and it will be her life on the ground and not my dads if their marriage was ruined.I could talk to him first but then once he knows I know then what? I'm sorry I'm offloading on here, I've told my partner but wish I hadn't, for a start he's been useless! I don't have anyone to talk to where our family won't end up being talk of the town. Thank you in advance for any replies

I'm very close to him, I think I can talk to him about it but not sure I want to get him involved so that my mum's the only person who doesn't know, assuming she doesn't! I've got some serious thinking to do

I think I'd be honest.And make sure even if you speak to your dad or brother first, that your mum does find out because I'd hate for it to come out years later and somebody says 'well gwil found out 6 years ago!' And you and your mums relationship suffers because you kept it from her. Does that make sense?

I have a half sister I have never met and like your half sister I messaged mine through FB. Like your half sister I never heard back. Not even a "I can't handle this, don't contact me" message. That really saddened me.

Our father is dead. I grew up without him. She grew up with him. I would have loved to hear a few stories of what he was like as a Dad. Only she could give me that. I didn't want to hurt her, or make her life difficult, I just wanted to find out about my own father and therefore my own identity. I have no siblings (or parents now) so I was excited to find a relative.

That's probably not helpful but I just wanted to give you a tiny insight into what might be her perspective.